High-carbon cold drawn steel filaments are known in the art. Cold drawing is applied to obtain the final diameter and to increase the tensile strength of the steel filament. The degree of drawing is, however, limited. The higher the degree of drawing, the more brittle the steel filament and the more difficult to reduce further the diameter of the steel filament without causing too much filament fractures. Commercially available wire rod diameters are typically 5.50 mm or 6.50 mm. Direct drawing from wire rod until very fine diameters is not possible.
The above-mentioned limited degree of drawing is the reason why the various drawing steps are alternated with one or more intermediate heat treatments. These heat treatments “reorganize” the internal metal structure of the steel filaments so that further deformation is possible without increase in the frequency of filament fractures. The heat treatment is mostly a patenting treatment, i.e. heating until above the austenitizing temperature followed by cooling the steel filament down to between 500° C. and 680° C. thereby allowing transformation from austenite to pearlite.
The prior art has provided several ways for carrying out the cooling phase and the transformation from autenite to pearlite.
The cooling phase or transformation phase may be carried out in a bath of lead or a lead alloy, such as disclosed in GB-B-1011972 (filing date 14 Nov. 1961). From a metallurgical point of view, this is the best way for obtaining a proper metal structure for enabling further drawing of the steel wire. The reason is that having regard to the good heat transfer between the molten lead and the steel wire, the transformation from austenite to pearlite is more or less isothermal. This gives a small size of the grains of the thus transformed steel wire, a very homogeneous metallographic structure and a low spread on the intermediate tensile strength of the patented wire. A lead bath, however, may cause considerable environmental problems. More and more, legislation is such that lead is forbidden because of its negative impact on the environment. In addition, lead may be dragged out with the steel wire causing quality problems in the downstream processing steps of the steel wire. Hence, since a number of years, there has been an increasing need to avoid lead in the processing of steel wires and to have alternative transformation or cooling methods.
EP-A-0 181 653 (priority date 19 Oct. 1984) and EP-B1-0 410 501 disclose the use of a fluidized bed for the transformation from austenite to pearlite. A gas which may be a combination of air and combustion gas fluidizes a bed of particles. These particles take care of the cooling down of the steel wires. A fluidized bed technology may give the patented steel wire a proper metal structure with fine grain sizes and a relatively homogeneous metallographic structure. In addition, a fluidized bed avoids the use of lead. A fluidized bed, however, requires high investment costs for the installation and high operating or maintenance costs.
The austenite to pearlite transformation may also be done in a water bath such as disclosed in EP-A-0 216 434 (priority date 27 Sep. 1985). In contrast with fluidized bed technology, water patenting has the advantage of low investment costs and low running costs. Water patenting, however, may give problems for wire diameters smaller than 2.8 mm. The reason is that the heat content of a steel wire is proportional to its volume and the volume of a steel wire is proportional to d2, where d is the diameter of the steel wire:heat content=C1×d2 
The surface of a wire is proportional to its diameter d:surface=C2×d
As a result, the cooling speed which is proportional to the surface and inversely proportional to the heat content, is inversely proportional to the diameter d:cooling velocity=(C2×d)/(C1×d2)=C3/d
The consequence is that fine steel wires are cooled too fast, which increases the risks for formation of bainite or martensite.
EP-0 524 689 (priority date 22 Jul. 1991) discloses a solution to the above-mentioned problem with water patenting. The cooling is done by two or more water cooling periods alternated with one or more air cooling periods. The cooling speed in air is not that high as in water. By alternating water cooling with air cooling the formation of bainite or martensite is avoided for steel wires with a diameter greater than about 1.10 mm. As with water patenting, this water/air/water patenting is cheap in investment and cheap in maintenance costs. However, a water/air/water patenting method also has its inherent limitations. A first limitation is that for very fine wire diameters, the smallest water bath may also cause risk for bainite or martensite formation. A second limitation is that the water/air/water patenting result in a metal structure which is too soft, i.e. with grain sizes which are greater than the grain sizes obtainable with lead patenting or with fluidized bed patenting. This soft structure is featured by a reduced tensile strength. In addition, the metallographic structure is not so homogeneous and the spread on the intermediate tensile strength of the patented wire may be high.
Cancelling all water baths and using only air patenting is an option with the advantage that the risk for formation of bainite or martensite is not existent or very limited. However, air patenting leads to even softer and more inhomogeneous metal structures than water patenting or water/air/water patenting.
The above prior art illustrates that there is a need for an environment friendly way of continuous and controlled cooling of steel wire which gives intermediate steel wires with a high intermediate level of tensile strength of the patented wire, a small grain size and a homogeneous metallographic structure.